1. Field of the Invention
The present invention relates to a method of manufacturing a monolithic inkjet printhead. More particularly, the present invention relates to a method of manufacturing a monolithic inkjet printhead, which can easily obtain a uniform ink flow path by controlling a shape and a size of the ink flow path.
2. Description of the Related Art
In general, an inkjet printhead is a device that ejects fine droplets of an ink onto desired positions of a recording medium to print data in predetermined colors. The inkjet printhead can be classified into two types according to an ejecting mechanism of the ink droplet. One of the types is a thermal driving inkjet printhead that generates bubbles in the ink using a thermal source and which ejects the ink droplet by the expanding force of the bubbles created, and the other is a piezoelectric driving inkjet printhead that ejects the ink droplet by applying pressure onto the ink due to a transformed piezoelectric material.
FIG. 1 shows a general structure of a thermal driving type inkjet printhead. Referring to FIG. 1, the inkjet printhead includes a substrate 10, a flow path forming layer 20 stacked on the substrate 10, and a nozzle layer 30 that is formed on the flow path forming layer 20. An ink feed hole 51 is formed on the substrate 10, and an ink chamber 53 in which the ink is filled, and a restrictor 52 that connects the ink feed hole 51 and the ink chamber 53, are both formed on the flow path forming layer 20. A nozzle 54, through which the ink is ejected from the ink chamber 53, is formed on the nozzle layer 30. In addition, a heater 41 that heats the ink in the ink chamber 53 and an electrode 42 that supplies the electric current to the heater 41, are also both disposed on the substrate 10.
The ink droplet ejecting mechanism in the thermal driving type inkjet printhead having the above structure will now be described in greater detail as follows. The ink is supplied from an ink storage (not shown) to the ink chamber 53 after passing through the ink feed hole 51 and the restrictor 52. The ink filled in the ink chamber 53 is heated by the heater 41 that is made of a resistance heating material in the ink chamber 53. Accordingly, the ink is boiled and a bubble is generated, and the generated bubble expands to compress the ink filled in the ink chamber 53. Thus, the ink in the ink chamber 53 is ejected from the ink chamber 53 through the nozzle 54.
The thermal driving type inkjet printhead having the above structure can be integrally manufactured using a photolithography process, and the manufacturing process is shown in FIGS. 2A through 2E. Referring to FIG. 2A, the substrate 10 of a predetermined thickness is prepared, and the heater 41 for heating the ink and the electrode 42 for supplying the electric current to the heater 41, are both formed on the substrate 10.
In addition, as shown in FIG. 2B, a negative photoresist is coated on the entire surface of the substrate 10 to a predetermined thickness, and the coated photoresist is then patterned using a photolithography process so as to surround the ink chamber 53 and the restrictor 52, such that the flow path forming layer 20 is then formed.
In addition, as shown in FIG. 2C, a sacrificial layer 60 is formed by filling in a space that is surrounded by the flow path forming layer 20 with a positive photoresist. Specifically, the positive photoresist is coated on the entire surface of the substrate 10 to a predetermined thickness, and then the coated photoresist is patterned using a photolithography process to form the sacrificial layer 60. Here, since the positive photoresist is coated generally using a spin coating method, an upper surface of the photoresist is not planar due to the centrifugal force used. That is, as represented by a chain line in FIG. 2C, the positive photoresist rises convexly near the sides of the flow path forming layer 20. When the positive photoresist, the upper surface of which is not a planar surface, is then patterned, an edge portion of the sacrificial layer 60 rises sharply upward.
As shown in FIG. 2D, the negative photoresist is coated on the flow path forming layer 20 and the sacrificial layer 60 to a predetermined thickness, and the photoresist is patterned using a photolithography process to form the nozzle layer 30 having the nozzle 54.
Next, as shown in FIG. 2E, the ink feed hole 51 is formed by wet etching a back surface of the substrate 10, and the sacrificial layer 60 is removed through the ink feed hole 51. The restrictor 52 and the ink chamber 53 are then formed on the flow path layer 20.
However, when the nozzle layer 30 is formed on the sacrificial layer 60 by coating the negative photoresist in the step shown in FIG. 2D, the protruded edge portion of the sacrificial layer 60 formed by the positive photoresist may react with a solvent in the negative photoresist so that the edge portion may be transformed or melted. If the transformation or melting of the sacrificial layer 60 is generated, a cavity 70 is formed between the flow path forming layer 20 and the nozzle layer 30 as shown in FIG. 2E.
FIG. 3 is a SEM picture showing a cross section of the conventional inkjet printhead. As shown in FIG. 3, the cavity is generated between the flow path forming layer 20 and the nozzle layer 30, thus the flow path forming layer 20 and the nozzle layer 30 cannot be completely adhered to each other.
As described above, according to the conventional method of manufacturing the inkjet printhead, the shape and the size of the ink flow path cannot be controlled and therefore, uniformity of the ink flow path cannot be ensured. Accordingly, the ink ejecting performance of the printhead is lowered. Also, since the flow path forming layer 20 and the nozzle layer 30 are not completely adhered to each other, the durability of the inkjet printhead is degraded.
In addition, in the step shown in FIG. 2D, the negative photoresist coated on the sacrificial layer 60 is patterned through an exposure process, a developing process, and a baking process. However, the exposure process affects the positive photoresist forming the sacrificial layer 60 under the negative photoresist, as well as the negative photoresist forming the nozzle layer 30. In addition, if the positive photoresist is irradiated by ultraviolet ray, a photosensitive material included in the photoresist is photolyzed and N2 gas is generated. The N2 gas expands in the baking process and pushes the nozzle layer 30, thus the nozzle layer 30 may be spatially transformed.
Accordingly, a need exists for a method for manufacturing a monolithic inkjet printhead which can obtain a uniform ink flow path by controlling a shape and a size of the ink flow path with greater precision.